I have a project where 8 Wagner 4.0 RH probes were placed in a 40,000 square foot concrete slab on grade. The slab thickness is 6 inches with a vapor barrier on top of soil. The probes were placed at 40% of the depth of the slab. The slab was placed approximately 15 to 20 years ago. The HVAC has been running for over a month with the ambient temperature at 75 degrees and the humidity between 50 and 60%.

6 of the 8 probes read in the 70% to 80% range and the other two probes read 99%. The two 99% readings are on opposite sides of the warehouse. Any ideas on why the probes would read 99%?

I've seen this in two instances. One where there was 2" of sand below slab and the vapor barrier and the sand never truly dried out. The other had 2" of sand and the gutter drains were being fed into the slab w/o a drain.

Well...no "sand" below the slab. We cored the slab at a couple of locations a week after the probes were installed to check the slab thickness and to see if there was a vapor barrier. Slab thickness was 5 7/8 inches and beneath the vapor barrier was "virgin soil" consisting of a silty sand. All of the probes are placed well within the interior of the slab, 50 to 75 feet from the perimeter.

40K feet, this is a warehouse? What was the place used for in the past? I've seen old warehouses in silicone valley that had trenches cut out to add conduit and never had the vapor barrier replaced. Also are there drains? I've seen places where trucks were being washed inside (like on a pier in San Francisco) and that was re-wetting the slab.

Actually 58,000 square feet and the previous use was a supermarket. There are no apparent trenches/drains installed before or after the concrete slab was placed in these areas.

To add more complexity to this issue. I replaced one of the probes and tracked the RH over several hours and then the next day. The RH started at 70% and then climbed up to 94% over several hours. The next day it read 99% and holds there.